The problem in heat exchangers in which the temperature difference between the incoming, initially cool, heat absorbing fluid and the heat transferring, that is, heat-releasing fluid, is very high is best illustrated in connection with the diagram of FIG. 1. A typically prior art heat exchanger is shown in FIG. 1, in which a hot heat releasing gas is passed across one surface of a heat exchange plate 1 in the direction of the arrow 2. Suitable flow ducts and the like have been omitted from the schematic showing of FIG. 1 for ease of illustration. The heat is transferred to a gas which flows in direction of the arrow 3. The ducts between the hot gases, arrow 2, and the to-be-heated gases, arrow 3, cross each other.
The heat releasing gas enters at a temperature of between about 900.degree. C. to 1100.degree. C. in the heat exchanger, and leaves the heat exchanger at a temperature of about 200.degree. C. to 250.degree. C. The gas to be heated is raised from about room temperature, for example from up to about 50.degree., to a temperature of between 800.degree. C. to 950.degree. C. In the explanation hereinafter, reference will be made simply to a "gas" although, of course, other fluid media may be used, and the "gas" may be a fluid gaseous medium, for example steam.
The temperature relationships above explained result in substantial temperature differences, and, for example, the corner 4 (FIG. 1) will have a temperature difference of over 1000.degree. C. occurring across the heat exchange plates or ducts. This situation arises since on the one side of the corner 4, the hot gas in flow 3 is applied and at the other side thereof, immediately adjacent thereto, the initially cold medium, see arrow 3, is passed. This substantial temperature difference places high stresses on the separating elements, and plate-type heat exchangers of known construction could not accept this temperature drop thereacross. The plates would, in the regions of substantial temperature difference, twist or bend, and forces which arose were so great that connections made by soldering, brazing, or welding would tear. It has been tried to solve the problem by use of particularly high-quality materials when making the plate heat exchanger components. Even heat exchanger components made of highest-quality material, resistant to high temperature, thermal shocks and thermal differences could not solve the problem entirely. Such high-quality materials usually additionally were very expensive, using alloys based on nickel or cobalt. The lifetime of cross-flow heat exchangers subjected to substantial temperature differences could be increased, but deformation, twisting and heat induced changes in dimensions of the heat exchanger components as well as tearing of connections could not be entirely avoided, particularly in the region of the critical corners 4, for example.